Low-speed mechanical aerator impeller

ABSTRACT

A low speed impeller of large size and pumping capacity comprises a conical shroud and 3, 4 or 5 flat blades projecting therefrom. The shroud and blades are disposed and proportioned to provide a wider operating range and are particularly adapted for fabrication of flat plates which are welded directly to the shroud.

United States Patent [191 Rooney et a].

[ Sept. 9, 1975 LOW-SPEED MECHANICAL AERATOR IMPELLER Inventors: ThomasC. Rooney, Waukesha;

Robert E. Crandall, Greendale, both of Wis.

Assignee: Rexnord Inc., Milwaukee, Wis.

Filed: Sept. 26, 1973 Appl. No.: 401,100

US. Cl. 261/91; 210/219; 210/220; 259/8; 259/108; 416/188 Int. Cl. BOIF7/22 Field of Search 259/8, 23, 24, 43, 44, 259/107, 108, 122; 416/188;261/92, 91; 210/220, 219

References Cited UNITED STATES PATENTS Durdin 210/8 2,346,366 4/1944Durdin 259/97 2,354,653 8/1944 Allen 3,323,782 6/1967 Clough 3,470,0929/1969 Bernard 210/220 X Primary Examiner-Harvey C. Hornsby AssistantExaminer.lames A. Niegowski [5 7] ABSTRACT A low speed impeller of largesize and pumping capacity comprises a conical shroud and 3, 4 or 5 flatblades projecting therefrom. The shroud and blades are disposed andproportioned to provide a wider operating range and are particularlyadapted for fabrication of flat plates which are welded directly to theshroud.

4 Claims, 6 Drawing Figures LOW-SPEED MECHANICAL AERATOR IMPELLERBACKGROUND OF THE INVENTION 1. Field of the Invention Fixed verticalaxis rotating single stirrer for fixed re ceptacle. Class 259/8.

2. Description of the Prior Art Impellers in general are designed forinducing an axial flow or a radial flow, the latter includingcentrifugal impellers. If the discharge is partially axial and partiallyradial, the impeller is a mixed flow impeller.

Impellers have some number of vanes or blades projecting from the hubwhich is mounted on the shaft which supports and rotates the impeller.

In large impellers the efficiency of the blades is generally improved bya shroud around the hub which blocks off the center space and directsthe flow smoothly past or around the hub. The hub and shroud may ofcourse comprise the same structure.

Each blade of an impeller has a boundary edge which includes the portionwhere it is attached to the hub or where it meets the shroud. Theboundary edge also has three other portions which are generally definedin terms of impeller rotation and the direction of the flow, viz. theleading edge, the inlet edge, and the outlet edge.

Other considerations which enter into the design of an impeller includeits balance, its configuration such that it is readily fabricated, andthe presence of strings and rags which may wrap themselves on theblades.

One method of aerating waste water or industrial wastes for itsbiological treatment is by the use of a ro tating impeller located atthe surface of the water. Such impellers are generally fixed on thelower end of the vertical output shaft of a motor driven gear reducerwhich is supported either by a fixed structure or by floats or pontoons.

A high speed aerator is one which generally includes an axial flowimpeller in a vertical draft tube with an upper member which deflectsthe water in a circular pattern having a considerable upward trajectory.In terms of pumping a given quantity of water per horsepower or currentconsumption, are of lower first cost but of less efficiency.

Low speed aerators are generally of two types and are driven by motorsor engines of up to 175 hp. One type churns large quantities of water atthe surface for aeration. The other type is designed to draw as muchwater as desired from as deep a level below the unit as is necessary toeffect mixing and scouring of the bottom of the basin in which the unitis operating. The water is then discharged horizontally to create alarge flow across the surface of the water toward the wall of the tank.This up-draft type in a fixed structure might include a large draft tubehaving an inlet near the floor of the tank. These units generallyinclude an axial flow impeller and the discharge may have a verticalcomponent similar to that of the high speed aerator. The use of anopen-style impeller without a draft-tube is preferred because of itsmore simple structure, and many open-style impellers are presentlyclaimed to be of the up-draft type but their effectiveness at depthsbelow feet is relatively modest. Additionally, slight change of theirposition relative to the water surface greatly affects their operation.Reference here is made to pumping capacity and submergence range andthese two factors are somewhat differently important to both the designof a floating mechanical aerator and to the design of a sewage or wastetreatment tank, pond or lagoon in which the units are mounted on piersor bridges.

In most sewage or waste treatment tanks the flow through the tank andthe elevation of the water level are controlled by effluent weirs whichoperate according to the formula K Q/L= V 8gH% where L is the weirlength and H is the water level above a straight, sharp crested weir.

The formula will show that at higher and lower flow rates (Q) the waterlevel will be higher and lower respectively, for a given weir length.

In a typical sewage treatment tank having fixed surface aerators thisrelationship may be utilized so that, for example, during the day and ata high flow rate, the impeller will be more supmerged and operate atmaxi mum capacity and during the night when the flow rate is greatlydiminished, the impeller will operate at reduced capacity because thelowered water level causes the impeller to be less submerged. Thisself-regulation can be utilized to great advantage in respect of thebiological processes of the organisms which utilize the oxygen which isbeing supplied to them. Additionally, of course, a saving in power costsis realized in the reduced power requirements at low flows, havingreference here to pumping efficiency as well as capacity.

However, the known prior art impellers exhibit a wide range of pumpingcapacity over a narrow range of submergence. This means that in terms ofoverall tank design, the tank must be provided with impossibly longweirs in order to hold the water level within the narrow range allowedby the impeller. Alternatively, mechanically operated weirs have beenprovided to maintain a constant level.

Such a device is of course the type of mechanical complication which theselection of maintenance-free low speed aerators would be intended toavoid, and ad ditionally. it can be seen that such a device which doesin fact maintain a constant liquid level entirely eliminates the severaladvantages of the self-regulating feature which have been described.

On the other hand, many industrial waste treatment systems operate withrelatively constant conditions over long periods of time and the lowspeed mechanical aerators may more readily be mounted on floats whichsupport the units at a given elevation relative to the water levelj Thedisplacement of each of these floats must not only be adequate tosupport each aerator but additionally must hold the unit steady againstthe downward reaction of the pumping effort. The pumping action isalways subject to some pulsing and the entire unit is susceptible tobobbing in the water. Imperceptible and unpredictable interactions ofsome number of units in the same tank, pond or lagoon will inexplicablycause one or some to rise and fall. When several fall in unison thecombined excess current draw may overload the electrical supply andactuate the protective circuit breaker. This is most likely to occurwhen one or some of the units are started up or shut down.

With reference to the impeller characteristics here, an impeller whichimposes a much larger torque load on the motor when the impeller is onlyslightly deeper in the water, is likely to exhibit some suchinstability.

The object of the present invention, thus, is to provide a more stableimpeller having the desired pumping capacity and submergence range whichis required for and characteristic of a typical biological treatmenttank utilizing surface aeration.

SUMMARY OF THE INVENTION The impeller of the present invention has a 45conical shroud and three, four or five flat blades which ex tenddownwardly therefrom so that their inlet edges are substantially belowthe apex of the shroud. The angle between the leading edge of each bladeand the impeller axis is in the order of 30. In general terms, theoverall height of the impeller is in the order of two-thirds of itsdiameter. A prime characteristic of the impeller is its wide bandsubmergence limits for a typical perfor mance range.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of arepresentative biological waste treatment tank with a bridge mountedmechanical aerator.

FIG. 2 is a central vertical cross section of the tank shown in FIG. 1.

FIG. 3 is a plan view of the blades of the impeller. That is, the viewis that of the blades looking down wardly. The drive shaft is shown insection and the outline of the shroud is shown in broken lines.

FIG. 4 is a side elevation of the impeller of FIG. 3.

FIG. 5 is a vertical elevation of one-half of the impeller taken on line55 of FIG. 3.

FIG. 6 shows a blade of the impeller with additional marginal portionswhich may be provided in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A representative square tank 8is shown in FIGS. 1 and 2 and includes the baffled inlet pipe 9 and theoutlet weir 10 which controls the elevation of the water level withinthe tank. One of the four side walls of the tank includes the weirtrough 11 which extends the length of weir 10 and includes the effluentpipe 12. The mechanical aerator 13 is mounted on the bridge 14 at thecenter of the tank and includes the upper drive motor 15, the gearreducer 16 having an output shaft 17 extending downwardly therefrom andbelow bridge 14 and the impeller 21 which is mounted on the lower end ofshaft 17 for rotation therewith at the surface of the tank contents.

Impeller 21 comprises the shroud 22, the several blades 23 and a hub,not shown in detail, by which the impeller is removably secured to shaft17. The hub and shroud 22 may be of any suitable construction. A featureof the invention resides in the fact that blades 23 are of flat plateswhich may be welded at their inner boundaries 24 directly to shroud 22.

Shroud 22 is in the form of a right circular 45 cone and as suchincludes a circular upper end which is bounded by the upper edge 22a.The height of the shroud from its lower apex 22b to its upper end isthus equal to the radius of the upper edge 22a. For practical reasons,the shroud may be truncated as shown such that it has a relatively smallcircular lower end 22c.

The inner boundary 24 of each blade has been referred to. As shown inFIGS. 3-5, the remaining outer boundary includes the leading edge 25,the inlet edge portions 26a and 26b, and the outlet edge 27. Each blade23 forms an angle in the order of 30 with the downward projection A ofthe axis of shaft 17 and the blades are equally spaced around the axis.The intersection of the plane of each blade 23 and the axis of theimpeller is located at the upper end of shroud 22 such that the planereferred to may be further defined as a plane which includes a radius Rof shroud '22 at its upper end and defines therewith an angle of 60.

Proceeding with such reference points, the leading edge 25 of blade 23extends from shroud 22 to a lower elevation approximately 033R belowapex 22b and the outlet edge 27 from edge 22a of shroud 22 to a pointapproximately at the elevation of apex 22b and radius R. The inlet edgeincludes two portions. Portion 26a extends horizontally at said lowerelevation from leading edge 25 and inlet edge portion 26b extends therefrom to the lower end of outlet edge 27 which has been defined.

The basic configuration of blades 23 has been shown and described forthe purpose of defining the invention and it should be understood thatthe actual outline of the blades may be curved or rounded by others withthe th) ught that some gain in efficiency may result. Also, FIG. 6 showsby way of example the additional areas which may be provided inaccordance with the invention. This may include the triangular portion31 such that the leading edge 25a has a slight negative rake respectingits rotation.

Where a given impeller design has been set up for manufacture and animpeller of some greater capacity is required, the trailing section 32may be added such that the outlet edge 27a extends to a radius of 1.2Ras shown, but not appreciably greater than l.5R. Impeller 21 could alsobe provided with four similar blades and possibly with five blades.However, the blades extend 90 or more around the shroud 22 and theoverlapping of the blades is thought to make six or more bladesimpracticable.

As has been mentioned, the usual tank, pond or lagoon is of such sizethat a considerable number of aerators are required at equally spacedlocations. The tank 8 may be considered typical of a biologicaltreatment tank as a single unit. The tank as shown is feet X 80 feet X20 feet and the water level to the weir 10 is 18 feet. Such a tank wouldaccommodate a normal flow (Q) for a typical waste in the order of 7,500gallons per minute. Minimum and maximum flows might be 2,500 gpm. and22,000 gpm. respectively.

For such flows the following tabulation is derived using the establishedformula given in the description of the prior art.

It will be seen from the above, that impeller 21 would require a weirlength of only 29 feet because of its submergence range. On the otherhand, to provide the same tank with the prior art impeller described, aweir length of somewhere between 80 feet and 120 feet may be required.While a weir is an especially inexpensive device, the necessity ofproviding a weir of such additional length for tank 8 would requireextending the weir trough 11 around the tank as shown in FIG. 2 by thebroken line 11a and would be a considerable complication.

The unexpected advantages of the present invention in relation to weirlength should of course not be considered in every case to be moreimportant than others which have also been mentioned or described atless length. The advantages such as in manufacturing simplicity areselfevident; the increased pumping effcicncy of the impeller which isclaimed, is presently indicated by applicants efforts in the developmentof the present invention. The stability of an impeller is gener allyonly based on observation or deduction after a power shut down hasoccurred.

We claim:

1. ln apparatus for aerating and mixing a body of water, said apparatuscomprising rotational drive means having a vertical output shaftextending downwardly therefrom, an impeller fixed to the lower end ofsaid shaft and having a lower inlet end, and means for supporting saiddrive means so that the upper end of the impeller is at a selectedelevation above the water surface and the lower end of the impeller isat a selected level of submergcncc; the improvement wherein saidimpeller comprises a conical shroud of R height and be tween three andlive flat blades, the larger upper end of the shroud defining the upperend of the impeller and having a radius R, each said blade being in theform of a flat plate which is set with a 30 rearward rake havingreference to the upward direction of the flow toward the propeller anddisposed so that its plane intersects the impeller axis at the upper endof the impeller, the perimeter of each blade consisting of l a partwhich adjoins the shroud, (2) a leading edge, (3) an inner inlet edge,(4) an outer inlet edge, and (5) an outlet edge, said leading edge ofeach blade extending from the shroud downwardly and radially outward tothe inner inlet edge ofthe blade, the outlet edge of each blade being ofR height and having a radius of between R and 1.5R and extending fromthe outer inlet edge to the upper outer edge of the shroud, the innerinlet edges of the blades being about 0.3R below the lower end of theshroud and defining the lower end of the impeller.

2. The improvement of claim 1 in such apparatus wherein the leading edgeof each blade has a slight radially rearward rake.

3. The improvement of claim 1 in such apparatus wherein the lower end ofthe shroud is truncated.

4. The improvement of claim 1 in such apparatus wherein the supportingmeans is adapted to support the drive means so that the upper end of theimpeller is in the order of 0.25R-0.2R above the level of the watersurface.

1. IN APPARATUS FOR AERATING AND MIXING A BODY OF WATER, SAID APPARATUSCOMPRISING ROTATIONAL DRIVE MEANS HAVING A VERTICAL OUTPUT SHAFTEXTENDING DOWNWARDLY THEREFROM, AN IMPELLER FIXED TO THE LOWER END OFSAID SHAFT AND HAVING A LOWER INLET END, AND MEANS FOR SUPPORTING SAIDDRIVE MEANS SO THAT THE UPPER END OF THE IMPELLER IS AT A SELECTEDELEVATION ABOVE THE WATER SURFACE AND THE LOWER END OF THE IMPELLER ISAT A SELECTED LEVEL OF SUBMERGENCE, THE IMPROVEMENT WHEREIN SAIDIMPELLER COMPRISES A CONICAL SHROUD OF R HEIGHT AND BETWEEN THREE ANDFIVE FLAT BLADSE, THE LARGER UPPER END OF THE SHROUD DEFINING THE UPPEREND OF THE IMPELLER AND HAVING A RADIUS R, EACH SAID BLADE BEING IN THEFORM OF A FLAT PLATE WHICH IS SET WITH A 30# REARWARD RAKE HAVINGREFERENCE TO THE UPWARD DIRECTION OF THE FLOW TOWARD THE PROPELLER ANDDISPOSED SO THAT ITS PLANE INTERSECTS THE IMPELLER AXIS AT THE UPPER ENDOF THE IMPELLER, THE PERIMETER OF EACH BLADE CONSISTING OF (U) A PARTWHICH ADJOINS THE SHROUD, (2) A LEADING EDGE, (3) AN INNER INLET EDGE,(4) AN OUTER INLET EDGE, AND (5) AN OUTLET EDGE, SAID LEADING EDGE OFEACH BLADE EXTENDING FROM THE SHROUD DOWNWARDLY AND RADIALLY OUTWARD TOTHE INNER INLET OF THE BLADE, THE OUTLET EDGE OF EACH BLADE BEING OF RHEIGHT AND HAVING A RADIUS OF BETWEEN R AND 1.5R AND EXTENDING FROM THEOUTER INLET EDGE TO THE UPPER OUTER EDGE OF THE SHROUD, THE INNER INLETEDGES OF THE BLADES BEING ABOUT 0.3R BELOW THE LOWER EDGE OF THE SHROUDAND DEFINING THE LOWER END OF THE IMPELLER.
 2. The improvement of claim1 in such apparatus wherein the leading edge of each blade has a slightradially rearward rake.
 3. The improvement of claim 1 in such apparatuswherein the lower end of the shroud is truncated.
 4. The improvement ofclaim 1 in such apparatus wherein the supporting means is adapted tosupport the drive means so that the upper end of the impeller is in theorder of 0.25R-0.2R above the level of the water surface.